1. Field of the Invention
The present invention relates generally to optical recording and/or reproducing apparatus, and more particularly, is directed to improvements in an apparatus operative to cause a main light beam and at least two auxiliary light beams to impinge upon a disc-shaped record medium for recording information thereon or reproducing information therefrom.
2. Description of the Prior Art
In an optical disc player for optically recording an information signal on or optically reproducing an information signal from an optical disc-shaped record medium having a center hole and a recording area on which a spiral record track is composed to surround the center hole, an optical head device is provided which comprises an optical arrangement for causing a light beam to impinge upon the recording area of the optical disc-shaped record medium rotating with a rotation center disposed at the center hole so as to form the spiral record track corresponding to a recording information signal on the recording area or to read an information signal recorded in the spiral record track on the recording area.
In the optical head device, for example, a laser light beam produced by a light beam generating portion is collimated by a collimator lens and enters into an objective lens to be focused thereby to impinge upon the optical disc-shaped record medium. Then, a reflected light beam emanating from the optical disc-shaped record medium is directed through the objective lens to a beam splitter to be changed in the direction of its optical axis thereby so as to enter into a photodetecting portion. A reproduced information signal, a focus error signal and a tracking error signal are produced based on a detection output of the reflected light beam from the photodetecting portion. A focus servocontrol operation for maintaining correct focus of the light beam projected onto the optical disc-shaped record medium and a tracking servocontrol operation for maintaining the light beam in correct tracking relation to each turn of the spiral record track on the optical disc-shaped record medium are performed in accordance with the focus error signal and the tracking error signal, respectively.
For, the production of the tracking error signal, various signal producing systems including the so-called "Push-Pull system" have conventionally been used. In the so-called "Push-Pull system", the reflected light beam emanating from the optical disc-shaped record medium is detected by a photosensor having a photodetecting element divided into two parts. The tracking error signal is produced based on a difference in level between detection outputs obtained from two parts of the photodetecting elements, respectively. In the case where the tracking error signal is produced in accordance with the so-called "Push-Pull system", although an optical arrangement for obtaining the tracking error signal and a signal processing circuit arrangement accompanying with the optical arrangement can be construed to be relatively simple, there is a disadvantage. That is, a light spot formed on the photosensor by the reflected light beam from the optical disc-shaped record medium is undesirably moved, regardless of the tracking condition of the light beam impinging upon the optical disc-shaped record medium and therefore the tracking error signal comes to contain undesirable DC offsets when the optical disc-shaped record medium is inclined in its radial direction to the optical axis of the objective lens through which the light beam is incident upon the optical disc-shaped record medium.
Accordingly, with the intention of avoiding the above mentioned disadvantage encountered with the so-called "Push-Pull system", an improved Push-Pull system has been proposed, as disclosed in the Japanese Patent application published as Laid-Open number 61-94246. In the case where the tracking error signal is produced in accordance with the improved Push-Pull system, at least two light beams which are obtained by dividing a light beam produced by a light beam source are caused to impinge upon an optical disc-shaped record medium for forming respective light beam spots on the same with a space therebetween in the radial direction of the optical disc-shaped record medium, which corresponds to a distance in a predetermined relation to the track pitch of the spiral record track. Then, each of two reflected light beams emanating from the optical disc-shaped record medium is detected by a photosensor having a photodetecting element divided into two parts. A difference in level between detection outputs obtained respectively from two parts of the photodetecting element of the photosensor which detects one of two reflected light beams is subjected to level adjustment for compensating a difference in strength between the two reflected light beams. The aforementioned difference is and then subtracted from a difference in level between detection outputs obtained respectively from two parts of the photodetecting element of the photosensor which detects the other of the reflected light beams to produce thereby the tracking error signal.
The tracking error signal thus obtained is prevented from containing DC offsets so as to represent properly the tracking condition of the light beam impinging upon the optical disc-shaped record medium, even if the optical disc-shaped record medium is inclined in its radial direction to the optical axis of the objective lens through which the light beam is incident upon the optical disc-shaped record medium.
When a light beam from an optical head device is caused to impinge upon an optical disc-shaped record medium which is rotating to form thereon a spiral record track corresponding to a recording information signal or to read an information signal recorded in a spiral record track formed thereon in an optical disc player, it is required for the light spot formed by the beam impinging upon the optical disc-shaped record medium to move relative to the optical disc-shaped record medium in a direction from the outermost fringe portion of the optical disc-shaped record medium to the innermost fringe portion of the optical disc-shaped record medium, or vice versa. Accordingly, it is the usual practice that the optical head device is moved in the direction from the outermost fringe portion of the optical disc-shaped record medium to the inner most fringe portion of the optical disc-shaped record medium, or versa. Consequently various head driving mechanisms for moving the optical head device in the direction from the outermost fringe portion of the optical disc-shaped record medium to the inner most fringe portion of the optical disc-shaped record medium, or vice versa, have been proposed.
One head driving mechanism that has been proposed is of a swing arm type in which a swing arm pivoted at one end is provided for supporting the optical head device attached to the other end thereof. The optical head device is moved in the direction from the outermost fringe portion of the optical disc-shaped record medium to the inner most fringe portion of the optical disc-shaped record medium, or vice versa when the swing arm is rotated on its pivoted end. Such a head driving mechanism of swing arm type can have a relatively simple construction and is advantageous in view of utilizing space 7 efficiently in the optical disc player.
However, in the event the optical disc player employs the head driving mechanism of swing arm type, a moving locus of the optical head device is along an arc of a circle with a center disposed at a position of the pivoted end of the swing arm. Therefore, the moving locus does not extend along the radial direction of the optical disc-shaped record medium. Consequently, when a plurality of light beams are incident upon the optical disc-shaped record medium to form respective light beam spots aligned thereon, a direction of the alignment of the light beam spots on the optical disc-shaped record medium is varied to a direction tangential to the spiral record track with the movement of the optical head device on the optical disc-shaped record medium.
As a result, in the case where the tracking error signal is produced in accordance with the improved Push-Pull system as described above in the optical disc player employing the head driving mechanism of swing arm type, the direction of the alignment of two light beam spots, which are formed on the optical disc-shaped record medium to be used for producing the tracking error signal, is varied to the direction tangential to the spiral record track with the movement of the optical head device on the optical disc-shaped record medium. The result is that the tracking error signal has level variations, irrespective of the tracking condition of the light beam impinging upon the optical disc-shaped record medium, in such a large scale as to interfere with the tracking servocontrol operation being carried out correctly.